Cisco UBR 7246 VXR - Multi ISP.

Trying to make sure I understand this properly....

You have one cable plant, that two different ISPs offer internet service on, is this correct?

if so, you can do two cards for each ISP, however this will use twice the spectrum, i.e. two docsis downstream channels and at least 2 upstream channels, if you have the spectrum available this is fine.

If you want to combine them, this will also work, assuming you use dhcp. If you use dhcpd you will need three separate dhcp servers 2 for the modems and one for the clients.

Using these values (substitute your real IPs in place)

ISP1

10.0.0.0/16 Modems
192.168.0.0/24 Clients

ISP2
10.1.0.0/16 Modems
192.168.1.0/24 Clients

On your cmts you set it up like so

bundle 1
no ip address
! this sends modem dhcp requests to two different servers and clients to another
cable helper-address 172.16.0.1 modems
cable helper-address 172.16.0.2 modems
cable helper-address 172.16.0.3 clients

bundle 1.1
ip address 10.0.0.1 255.255.0.0
ip address 192.168.0.1 255.255.255.0 secondary

bundle 1.2
ip address 10.1.0.1 255.255.0.0
ip address 192.168.1.1 255.255.255.0 secondary

Modem DHCP server
ISP1 172.16.0.1

shared-network ModemPool {
# pool for Modems for ISP 1
subnet 10.0.0.0 mask 255.255.0.0 {
routers 10.0.0.1;
# other necessary ip info
pool {
range 10.0.0.2 10.0.255.254;
}
}
}

# host entries for authorized modems for this ISP
host client1 {
hardware 1:0:0:0:0:0:1;
}
dhcpd server for modems
ISP2 172.16.0.2

shared-network ModemPool {
# pool for Modems for ISP 2
subnet 10.0.0.0 mask 255.255.0.0 {}
subnet 10.1.0.0 mask 255.255.0.0 {
routers 10.1.0.1;
# other necessary ip info
pool {
range 10.1.0.2 10.1.255.254;
}
}
}

#Modems authorized for this isp
host client2 {
hardware 1:0:0:0:0:0:2;
}

On your Dhcpd server for clients 172.16.0.3

shared-network ISP1 {
subnet 10.0.0.0 mask 255.255.0.0 {}
subnet 192.168.0.0 mask 255.255.255.0 {
routers 192.168.0.1;
name-servers 192.168.0.2;
pool {
range 192.168.0.3 192.168.0.254;
}
}
}
shared-network ISP2 {
subnet 10.1.0.0 mask 255.255.0.0 {}
subnet 192.168.1.0 mask 255.255.255.0 {
routers 192.168.1.1;
name-servers 192.168.1.2;
pool {
range 192.168.1.3 192.168.1.254;
}
}
}

The way this works is when the modem dhcps it uses the first ip on the bundle interface in this case 10.0.0.1 as the source IP for the dhcp request, it sends it to both modem dhcpd servers 172.16.0.1 and 172.16.0.2, if the modem is authorized it will get an IP from one or the other. When the client sends its dhcp request it will forward the request based on which ip the modem has, i.e. if the modem has an ip of 10.0.0.2 it will forward the request from the 10.0.0.1 interface, if the modem has a 10.1.0.3 ip address it will forward the request from the 10.1.0.1 interface, when this hits the clients dhcpd server it will automatically choose the right subnet and assign the correct ip.

I use this approach in my CMTS setup except in a Authorized Not Authorized situation instead, it works well, but is a little tricky to set up. We also use one physical server that is virtualized into multiple servers, using Xen to save on hardware.